Open Systems are systems designed to incorporate all devices regardless of manufacturer and accept third party add-on hardware or software products. Industry standards fall into two categories: de jure and de facto.
De jure standards have been created by standards bodies, such as the American National Standards Institute (ANSI), the Institute of Electrical and Electronic Engineers (IEEE), and the International Standards Organization (ISO). For example, the ANSI American Standard Code for Information Interchange (ASCII) character encoding standard, the IEEE Portable Operating System Interface for UNIX (POSIX) standard, and the ISO Open System Interconnection (OSI) reference model for computer networks are all de jure standards.
De facto standards have been widely adopted by industry but are not endorsed by any of the standards bodies. An example of a de facto standard is the Transmission Control Protocol/Internet Protocol (TCP/IP) network communications protocol. De facto standards exist either to fill gaps left by the implementation specifications of the de jure standards or because no standard currently exists for the particular area.
Open systems based solely on de jure industry standards do not yet exist and probably never will because of the very different natures of the computer industry and the academic-standards process. The speed with which the technology changes is staggering, and the formal standards process can't keep pace with it. Various composites of system standards exist because the market demands that solutions be implemented immediately. In today's open systems, both de facto and de jure standards are combined to create interoperable systems. It is the strategic combination of both types of standards that enables open systems to keep pace with rapidly changing technology.
One key element of this middle-of-the-road approach is the use of strategically placed layers of software, to allow the adjoining upper and lower layers of software within the operating system to provide different functions that work together. These software layers provide a standardized set APIs to the software layers above and below themselves. A good example is the Network Device Interface Specification (NDIS), which was jointly developed by Microsoft and 3Com in 1989. Another example is the Desktop Management Interface (DMI) created by the Desktop Management Task Force (DMTF), an industry organization with more than 300 vendor members, including Microsoft, Intel, IBM, and Novell.
The benefit of this architecture is that it allows software modules above and below the layer to be substituted for software modules developed using the same standards. This means you can start out with a module that implements a de facto standard and later supplement or replace it with one that implements a de jure standard. In effect, you end up with the best of both worlds — open systems and industry standards.